The present invention relates to tanks for storing liquids and more particularly to tanks for storing liquids which can freeze at low ambient temperatures.
It is well known to store large quantities of liquids in both aboveground and underground tanks, especially liquids produced from such industries as the oil and gas industry, where liquids such as water contaminated with oil, must be stored on site before removal and cleanup. Liquid storage is also required in a number of different industries and applications.
Aboveground tanks are often preferable to underground tanks as there is no need to excavate a site and leakage detection is more easily performed. Regulations governing environmental protection, hazardous materials handling and worker safety provide structured guidelines with which such storage tanks can be constructed, whether single-walled or double-walled.
As taught in U.S. Pat. No. 5,971,009 to Schuetz et al., the use of aboveground tanks in climates subject to extreme ambient temperatures has not found favor in the industry, due to problems such as freezing or increased viscosity of tank contents. Schuetz et al. addressed the freezing problem by providing a support means upon which the tank was placed, so as to create an air space under the tank. The entire structure and the air space is isolated from ambient using a layer of insulation. Further, a heater is used to heat the air space below the tank to keep the tank""s contents from freezing, rather than heat the content""s of the tank directly, which was deemed to be expensive and impractical. Heat can also be directed into the annular space formed between the inner and outer walls.
The above prior art is in the form of a custom constructed tank. Construction of such aboveground tanks requires a significant amount of cost and man-hours. In times of increased activity in industries such as the drilling and production sector of the petroleum industry, it may be difficult to supply the large number of tanks required to satisfy needs. Any additional complex construction for integrating tanks, support means and heaters into complete, heated-tank systems increases the amount of time and money required to produce tanks. Further, advance construction and stockpiling of tanks is often not a practical solution, as it is difficult to predict their use in many industries which have fluctuating needs, resulting in a large amount of revenue being tied up and unrecoverable until the tanks are sold.
Further, most well sites do not have ready access to electrical power, if any, and therefore it is known to utilize equipment capable of being run using well products such as raw natural gas.
Ideally, a heating system for a liquid storage tank, whether part of the original design of a tank system or as a retrofit to an existing tank system, should be relatively inexpensive to build and to operate, provide adequate heat to the tanks contents to prevent freezing, require no electricity, be easily accessible from the exterior of the tank system for servicing and preventative maintenance, utilize simplified construction and be easily added to existing tank systems.
The present invention provides a heating system that is simple to construct and is readily retrofit to existing tank systems. The heating system satisfies the requirements of being readily accessible for service and maintenance, and does not require electrical power to operate.
In a broad aspect of the invention, a tank heater system is provided comprising a hollow heating chamber having a lower inlet, in communication with an inlet line extending into and adjacent the bottom of the tank, and an upper outlet, in communication with the liquid in the tank. A heater is positioned for heating the heating chamber. Liquid, drawn from the tank into the inlet line, is heated in the heating chamber where it rises by convection and is reintroduced to the tank through the outlet.
Preferably, the heating chamber has a plurality of baffles inside the hollow chamber for increasing the residence time of the liquid in the heating chamber and increasing the fluid""s heat capacity. A flameless, infrared gas catalytic-type heater can be used to avoid the need for electricity and comply with explosion proof conditions. Further, the discharge to the tank is in constant communication with the liquid in the tank, including the use of a floating discharge which remains in constant communication with the liquid in the tank and thus preventing airlock when the liquid level drops below that of the upper outlet""s connection to the tank. Enclosing the heating system against the tank wall, scavenges residual heat and applies it to the tank. In yet another embodiment of the invention, a gas powered or heat powered pump is fitted into heating chamber system, thereby creating forced convection to ensure liquid flow is maintained.